Myocardium & Cardiomyocytes

At medium magnification (100x) with Masson’s trichrome stain, the myocardium is composed of bundles of cardiac muscle fibers that stain a deepAway from the surface of the body. red due to their actin and myosin content. Each fiber stains a deep red, reflecting its rich contractile protein content, and they appear elongated, cylindrical, and often branching, weaving into a syncytial network. Their boundaries are outlined by thin streaks of blue connective tissue, where delicate collagenA structural protein in the dermis that provides strength and elasticity. fibers provide structural support and house capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues..

The fibers are elongated and branching, often running in slightly different orientations, giving the tissue a woven appearance. Individual fibers are separated by delicate strands of connective tissue, which appear blue in this stain, and within these spaces run capillaries and small vessels that supply the muscle.

The nucleiClusters of neurons in the CNS responsible for processing information. of the cardiac myocytes are centrally located, oval, and stain dark purple. In some fibers, the cross-striations may be faintly visible, but at this magnification, the most striking features are the branching fibers and the intercalated discs Structures in cardiac muscle that allow electrical connectivity.. These intercalated discs appear as thin, dark, irregular lines running perpendicular to the muscle fibers and mark the sites of specialized junctions that electrically and mechanically link the cellsThe basic structural and functional units of life. together.

Compared with the dense, acellular collagen of the fibrous skeleton, the myocardium has a rich vascular supply. This view under trichrome highlights the myocardium’s contractile role in generating force, supported by the connective tissue scaffolding that transmits and coordinates the pull of each fiber.

The nuclei of cardiomyocytes are typically central within the fibers, oval in shape, and stain a dark purple. Unlike skeletal muscle, which has multiple peripheral nuclei, most cardiomyocytes contain only one or two centrally placed nuclei, an identifying feature at this magnification. Intercalated discs are seen as fine, irregular, transverse dark lines across the fibers—though not all are visible in every plane of section A cut or slice of the body or an organ for study., they are distinctive markers of cardiac muscle tissue.

Heart, ventricle, H&E, 40X  Slide 098-1 Right heart wall, H&E, 40X
All by University of Michigan Histology, licensed under CC BY-NC-SA 3.0.